Ellsworth Avenue in Silver Spring, Md., sizzles on a summer evening: Residents stroll along its tree-lined sidewalks under the neon glow of upscale chain restaurants, retailers and a Whole Foods Market. Beyond the din of downtown is a different kind of sizzle: the intoxicating aroma of sliced meat frying in butter, garlic and onion—called tibs—wafting from one of many Ethiopian restaurants just outside the city center.